1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus of the type effecting photometry with a solid-state image sensor and a photometric method for the same.
2. Description of the Background Art
A digital camera, for example, is configured to set up pickup parameters, including a shutter speed, an F number, and a distance of the subject from the camera, to pick up a desired subject field in an optimum condition. In order to set up the parameters, the camera executes photometry, prior to capturing the image of a subject field. First, the user operates the camera, before a shot, to cause photometry and range finding in terms of the parameters. More specifically, the camera determines whether or not the quantity of light metered by photometry is adequate and whether or not the focussing resultant from range finding is adequate.
As for photometry, it is likely, in the worst case, that the photometric output saturates due to an excessive quantity of light or a signal cannot be sensed at all due to a short quantity of light. In such a case, the level gain of the photometric output signal is adjusted in accordance with the input signal level. In this manner, the first adjustment is effected such that the preselected quantity of light is incident to the photosensitive array of the camera. This is done by adjusting exposure parameters included in the pickup parameters of the camera. Subsequently, the camera again executes photometry with the adjusted exposure parameters and again determines whether or not the quantity of incident light is adequate. The camera repeats the above-mentioned procedure until it attains adequate values of the exposure parameters. As for range finding, too, the camera repeats a decision on focus error until the focus error becomes zero. The procedure for automatically executing the above sequence of steps is generally referred to as an AE (Automatic Exposure)/AF (Automatic Focusing) control procedure.
One of the problem with the AE/AF procedure is that a substantial period of time is necessary for attaining the adequate pickup parameters by repeated processing, resulting in possibly missing a desired scene. Should priority be given only to desired timing for shooting a particular scene, the camera would pick up the scene with the values of pickup parameters deviated from adequate ones and would therefore make the resulting picture unsatisfactory.
Japanese patent laid-open publication No. 207376/1993 proposes a solid-state image pickup apparatus and a control method for the same in which the individual photosensitive cell, or photodiode, is divided into a plurality of regions. In this prior art scheme, a switching device alternatively selects either one of the regions of the individual photosensitive cell to thereby vary the area to which light is incident and therefore the sensitivity of the cell or pixel. More specifically, the apparatus is configured to reduce a photosensitive area for a relatively lighter subject or increase it for a relatively darker subject for thereby adjusting the sensitivity. This, according to the above document, reduces the AE/AF processing time while broadening the dynamic range.
Further, the document mentioned above describes that the sensitivity is switched in accordance with information representative of the lightness of a subject at the same time as the AF control. A photometry device mounted on the camera for outputting the above information for AE control may have photosensitive cells arranged on a single chip together with a sensor for use in switching the sensitivity, so that the sensitivity can be switched in accordance with lightness information. However, the sensitivity switching sensor and a sensor for photometry mounted on a single chip are not accurately coincident with each other in the direction or the range of photometry and therefore lack accuracy.
In light of the above, to select an appropriate region of the individual photosensitive cell, i.e. appropriate sensitivity accurately matching with a subject, the level of a signal output from the sensitivity switching sensor is monitored to determine the lightness with accurate sensitivity. It is to be noted that to select particular sensitivity from the individual photosensitive cell means to execute photometry control with that cell. At this instant, every time the sensitivity of the photosensitive cell is switched between high and low, the sensitivity is reset; when a subject is relatively lighter, integration is effected two consecutive times. The document describes that because each time of integration ends in a short period of time, the entire integration for setting the sensitivity can complete in a short period of time.
As stated above, the procedure described above executes integration two times with high and low sensitivity, i.e. executes photometry by repeating exposure twice and reading out signal charges twice in total. Although the document teaches that the entire photometry time should only be shorter than the maximum integration time, exposure must eventually be executed once or twice in accordance with the lightness of a subject to be imaged. The number of times of exposure that is changeable in dependent on the lightness of a subject is undesirable when it comes to the reduction of photometry time.